During the normal life of an individual many of the tissues of the body are being continuously renewed, or replaced at definite periods. The replacement of a part may go on by a process of continuous growth, such as takes place in the skin and nails of man, or the replacement may be abrupt, as when the feathers of a bird are moulted. It is the latter kind of process that is generally spoken of as physiological regeneration. In the same animal, however, certain organs may be continually worn away, and as slowly replaced, and other organs replaced only at regular intervals. Bizozzero has made the following classification of the tissues of man, on the basis of their power of physiological regeneration. (1) Tissues made up of cells that multiply throughout life, as the parenchyma cells of those glands that form secretions of a definite morphological nature; the tissues of the testes, marrow; lymph glands, ovaries; the epithelium of certain tubular glands of the digestive tract and of the uterus; and the wax glands. (2) Tissues that increase in the number of their cells till birth, and only for a short time afterward, as the parenchyma of glands with fluid secretions, the tissues of the liver, kidney, pancreas, thyroid, connective tissue, and cartilage. (3) Tissues in which multiplication of cells takes place only at an early embryonic stage, as striated muscles and nerve tissues. In these there is no physiological regeneration. There are many familiar cases of periodic loss of parts of the body. The hair of some mammals is shed in winter and in summer. Birds renew their feathers, as a rule, once a year. Snakes shed their skin from time to time. The antlers of deer are thrown off each year, and new ones formed accompanied by an increase in size and branching of the antlers. In other cases similar changes may be associated with certain stages in the life of the animal. The milk-teeth of the mammals are lost at definite periods, and new teeth acquired.[61] The larval exoskeleton of insects is thrown off at intervals, and after each moult the body increases in size; but after the pupa stage is passed and the imago formed, there is no further moulting. In the crustacea, on the other hand, the adult animals moult from time to time, and the upper limit of size is less well defined than in the insects. The larvæ also pass through a series of moults.